The present invention relates to an electron gun for a CRT, and more particularly to an electron gun for a CRT having an improved method for applying voltage to respective electrodes constituting the electron gun.
Usually, a CRT, such as that shown in FIG. 1, comprises a panel P on the inner side of which a shadow mask assembly SF is mounted, and a funnel adhered to the panel in which an electron gun G is installed in the neck N of the funnel and a deflection yoke D is installed on the outer side of the neck.
In such a CRT, even if all three electron beams emitted from the electron gun are optimally focused on the center of a phosphor layer formed on the inner side of panel P, the electron beams deflected toward the periphery of the phosphor layer by deflection yoke D differ among one another in their focal distances, when landed on the phosphor layer. This is due to the geometric curvature of the inner surface of panel P and the fact that the electron gun is disposed in an in-line form. Also, when the electron beams are deflected toward the periphery of the phosphor layer, the cross sections of the electron beams are distorted due to the non-uniform magnetic field of the deflection yoke, so that the cross sections of the electron beams cannot be formed regularly throughout the phosphor layer. This causes the reduction of the CRT's resolution.
FIG. 2 illustrates a conventional electron gun for a CRT which solves the above problem. Referring to FIG. 2, the electron gun comprises cathodes 2, a control electrode 3, a screen electrode 4 which form a front triode, a focus electrode 5, a dynamic focus electrode 6 and an anode electrode 7 which concentrate and accelerate electron beams. Vertically-elongated electron beam passing holes 5H and horizontally-elongated electron beam passing holes 6H are formed on projection side 5a of focus electrode 5 and incidence side 6a of dynamic focus electrode 6, respectively. Focus voltage Vf and anode voltage Ve are applied to focus electrode 5 and anode electrode 7, respectively. Dynamic focus voltage Vd, which, taking focus voltage Vf as a base voltage, is varied according to the vertical and horizontal synchronous signals of the deflection yoke, is applied to dynamic focus electrode 6.
In the above electron gun for a CRT, when electron beams emitted from cathodes 2 are deflected toward the periphery of the phosphor layer, dynamic focus voltage Vd is applied to dynamic focus electrode 6 to form a quadruple lens between focus electrode 5 and dynamic focus electrode. Specifically, due to vertically-elongated electron beam passing holes 5H formed on projection side 5a of focus electrode 5 and horizontally-elongated electron beam passing holes 6H formed on incidence side 6a of dynamic focus electrode 6, a relatively weak condenser lens and a relatively intense divergent lens as compared with those in the horizontal direction, are formed vertically. Meanwhile, a relatively intense condenser lens and a relatively weak divergent lens as compared with those in the vertical direction, are formed horizontally. The electron beams passing through the lenses converge horizontally and diverge vertically so that their cross sections become vertically elongated. Thus, the distortion of the electron beams can be compensated for by a non-uniform magnetic field formed when the electron beams are deflected toward the periphery of the phosphor layer by the deflection yoke, so that a circular spot of the electron beams can be obtained on the periphery of the phosphor layer.
Further, since focus voltage Vf and dynamic focus voltage Vd are mixedly applied to dynamic focus electrode 6, the potential difference between the focus electrode and anode electrode 7 becomes small to reduce the intensity of a main lens formed therebetween and to elongate the electron beams' focal length, so that the electron beams are focused on the periphery of the phosphor layer in a favorable form.
The conventional electron gun, as described above, can compensate for the electron beams' focus characteristic and astigmatism. However, the conventional electron gun has a poor withstand voltage because a high voltage is applied to dynamic focus electrode 6. Withstand voltage is defined as the maximum voltage which can be safely supplied to electrodes without causing the destruction of insulation in connection with the electrodes. The circuit for applying the high dynamic focus voltage is difficult to construct. Furthermore, since the variation of the focal length of the electron beams emitted from the cathodes and the shaping of the electron beams' cross sections are carried out near the high voltage portion adjacent to a final accelerating electrode for finally accelerating the electron beams, when the dynamic focus voltage is applied, convergence drift, in which convergence for focusing three electron beams is varied, occurs to reduce a convergence characteristic. Convergence drift is herein defined as the deviation of the focusing of the electron beams on the electron beam passing hole.
To overcome these problems, a low voltage driving method is adapted, in which a voltage for controlling the focusing distance of electron beams from the electron gun to the phosphor layer is lower than a fixed focus voltage. However, this method is disadvantageous since it greatly reduces the compensating effect of the focus characteristic.